1. Technical Field
This invention relates to a surface acoustic wave filter which is used for a communication device, etc., such as a portable telephone.
2. Background Art
In recent years, along with development of mobile communications, realization of high performance and miniaturization of devices to be used have been desired. As a filter for a mobile communication device among those devices, conventionally, a surface acoustic wave filter (hereinafter, called a SAW filter) has been used widely. At the present day, as a SAW filter for an RF (Radio Frequency) stage, a longitudinal mode type and a ladder mode type are mainly used. In particular, as to the ladder type SAW filter, it is possible to realize a low loss as compared with the longitudinal mode SAW filter. The ladder type SAW filter is of such a configuration that a plurality of surface acoustic wave resonators (hereinafter, called SAW resonators) are connected in a ladder mode, and configured by SAW resonators operating as a serial arm and SAW resonators operating as a parallel arm.
As prior art relating to the ladder-type SAW filter, JP-A-9-270663 publication discloses a method of arranging SAW resonators operating as a serial arm and SAW resonators operating as a parallel arm, and separating them by a certain distance. FIG. 20 is a block diagram of a SAW filter configured in this manner and disclosed in the above-mentioned first prior art document. This SAW filter is configured by SAW resonators 902 as a serial arm, SAW resonators 903 as a parallel arm which are formed on piezoelectric substrate 901 and signal lines 904 connected to these SAW resonators 902, 903. These SAW resonators 902, 903 are arranged in such a manner that respective overlap lengths of comb-shaped electrodes configuring these SAW resonators 902, 903 overlap in a propagation direction of leaking surface acoustic wave. In this case, if it is set up in such a manner that a distance of a gap between SAW resonator 902 as the serial arm and SAW resonator 903 as the parallel arm becomes 10 times wavelength of the leaking surface acoustic wave, or more, it is possible to prevent the interference of a leaking surface acoustic wave in these SAW resonators 902, 903.
As another prior art relating to the ladder type SAW filter, JP-A-9-232908 publication discloses a method of arranging a slit plate between SAW resonators operating as a serial arm and SAW resonators operating as a parallel arm. FIG. 21 is a block diagram of a SAW filter of the above-mentioned second prior art document, which was configured in this manner. This SAW filter is configured by SAW resonators 1002, 1005 of a serial arm, SAW resonators 1003, 1006 of a parallel arm and slit plates 1004, 1007, which are formed on piezoelectric substrate 1001. Slit plate 1004 is disposed between SAW resonator 1002 of the serial arm and SAW resonator 1003 of the parallel arm, to block off surface acoustic waves leaking from respective SAW resonators 1002, 1003. In the same manner, slit plate 1007 is disposed between SAW resonator 1005 of the serial arm and SAW resonator 1006 of the parallel arm, to block off surface acoustic waves leaking from respective SAW resonators 1005, 1006.
As another prior art relating to the ladder type SAW filter, JP-A-2000-201052 publication discloses a method of arranging in such a manner that propagation paths of surface acoustic wave in SAW resonators operating as a serial arm and SAW resonators operating as a parallel arm do not overlap. FIG. 22 is a block diagram of a SAW filter of the above-mentioned third prior art document, which was configured in this manner. This SAW filter is configured by SAW resonators 1102, 1103 of a serial arm, and SAW resonator 1004 of a parallel arm, which are formed on piezoelectric substrate 1101. In this configuration, the filter is formed in such a manner that a surface acoustic wave propagation path of SAW resonator 1104 of the parallel arm, which is sandwiched by two SAW resonators 1102, 1103 of the serial arm, is located between surface acoustic wave propagation paths of these SAW resonators 1102, 1103 of the serial arm. By this means, surface acoustic waves of respective SAW resonators of the serial arm and the parallel arm do not interfere with each other, and a good filter characteristic is obtained.
However, in the above-mentioned conventional disclosure examples, SAW resonators of a serial arm and SAW resonators of a parallel arm are arranged in an isolated manner by a certain distance. On this account, there is a problem that a size of a SAW filter becomes large. These SAW filters disclose a technique of eliminating an interference of surface acoustic surface waves of SAW resonators of a serial arm and SAW resonators of a parallel arm, but do not disclose a way for utilizing this at all.
The present invention aims to provide a SAW filter which is of a small size and enables low loss; by a SAW filter with a new configuration such that a plurality of SAW resonators are arranged in proximity to each other on the same propagation path, while avoiding the above-mentioned problem.